In recent years, an exhaust gas purifying catalyst, in which particles of noble metal such as platinum (Pt) and rhodium (Rh) are supported on a support of metal oxide such as alumina (Al2O3), has been being widely used in order to remove harmful substances such as a hydrocarbon (HC)-based compound, carbon monoxide (CO) and nitrogen oxide (NOx), which are contained in exhaust gas discharged from an internal combustion engine. In the conventional exhaust gas purifying catalyst, a large amount of such noble metal particles is used in order to enhance durability of the noble metal particles against variations of an ambient atmosphere. However, it is not desirable to use the large amount of noble metal particles from a viewpoint of protecting the earth's resources.
One method for reducing such a usage amount of the noble metal while maintaining purification performance for the exhaust gas at a certain level or more is to reduce a particle diameter of the noble metal particles. If the particle diameter of the noble metal particles is reduced, then a specific surface area thereof is increased. Accordingly, it is sufficient that the usage amount of noble metal for obtaining desired purification performance of the catalyst is small. However, when the particle diameter of the noble metal particles is small, the noble metal particles mutually cause thermal aggregation (sintering) owing to a high-temperature use thereof and a long-term use thereof, and accordingly, there is an apprehension that the durability of the noble metal particles may be decreased.
In this connection, an exhaust gas purifying catalyst has been developed, which has a structure in which the noble metal particles are supported on first compounds, and the first compounds supporting the noble metal particles thereon are surrounded by second compounds, whereby the first compounds, on which the noble metal is supported, are separated from one another by the second compounds (Patent Document 1). In the exhaust gas purifying catalyst having such a structure, the noble metal particles are supported on the first compounds, and the noble metal particles are thereby chemically fixed to the first compounds, whereby movement and aggregation of the noble metal particles are suppressed. Moreover, the first compounds, which support the noble metal particles thereon, are physically separated from one another by the second compounds, whereby the first compounds, which support the noble metal thereon, are suppressed from mutual contact and aggregation. In such a way, the noble metal particles are prevented from being aggregated after durability test, whereby the durability thereof can be enhanced.